Grinding process and Type of Grinding
China precision machining Grinding is a manufacturing process that involves the removal of metal by employing a rotating abrasive wheel. The latter simulates a milling cutter with an extremely large number of miniature cutting edges.
Generally, grinding is considered to be a finishing process that is usually used for obtaining high-dimensional accuracy and better surface finish. Grinding can be performed on flat, cylindrical, or even internal surfaces by employing specialized machine tools, which are referred to as grinding machines.
Obviously, grinding machines differ in construction as well as capabilities, and the type to be employed is determined mainly by the geometrical shape and nature of the surface to be ground, e.g., cylindrical surfaces are ground on cylindrical grinding machines.
Type of Grinding Operations
1. Surface grinding. As the name surface grinding suggests, this operation involves grinding of flat or plane surfaces. Fig.8.1 indicates the two possible variations, either a horizontal or vertical machine spindle.
In the first case (horizontal spindle), the machine usually has a planer-type reciprocating table on which the workpiece is held. However, grinding machines with vertical spindles can have either a planer type table like that of the horizontal-spindle machine or a rotating worktable.
Also, the grinding action in this case is achieved by the end face of the grinding wheel, contrary to the case of horizontal-spindle machines, where the workpieces ground by the periphery of the grinding wheel.
During the surface-grinding operations, heavy workpieces are either held in fixtures or clamped on the machine table by strap clamps and the like, whereas smaller workpieces are usually held by magnetic chucks.
2. Cylindrical grinding. In cylindrical grinding, the workpiece is held between centers during the grinding operation, and the wheel rotation is the source and cause for the rotary cutting motion, In fact, cylindrical grinding can be carried out by employing any of the following methods:
(1) The transverse method, in which both the grinding wheel and the workpiece rotate and longitudinal linear feed is applied to enable grinding of the whole length. The depth of cut is adjusted by the cross feed of the grinding wheel into the workpiece.
(2) The plunge-cut method, in which grinding is achieved through the cross feed of the grinding wheel and no axial feed is applied. As you can see, this method can be applied only when the surface to be ground is shorter than the width of the grinding wheel used.
(3) The full-depth method, which is similar to the transverse method except that the grinding allowance is removed in a single pass. This method is usually recommended when grinding short rigid shafts.
3. Internal grinding. Internal grinding is employed for grinding relatively short holes, The workpiece is held in a chuck or a special fixture. Both the grinding wheel and the workpiece rotate during the operation and feed is applied in the longitudinal direction.
Any desired depth of cut can be obtained by the cross feed of the grinding wheel. A variation from this type is planetary internal grinding, which is recommended for heavy workpieces that cannot be held in chucks.
In that case, the grinding wheel not only spins around its own axis but also rotates around the centerline of the hole that is being ground.
4. Centerless grinding. Centerless grinding involves passing a cylindrical workpiece, which is supported by a rest blade, between two wheels, i.e., the grinding wheel and the regulating or feed wheel.
The grinding wheel does the actual grinding, while the regulating wheel is responsible for rotating the workpiece as well as generating the longitudinal feed. This is possible because of the frictional characteristics of that wheel, which is usually made of rubber-bonded abrasive.
the axis of the regulating wheel is tilted at a slight angle with the axis of the grinding wheel. Consequently, the peripheral velocity of the regulating wheel can be resolved into two components, namely, workpiece rotational speed and longitudinal feed.
These can be given by the following equations:
Axial feed＝Vregulating wheel×c×sinα
Where c is a constant coefficient to account for the slip between the workpiece and the regulating wheel (c=0.94~0.98).
The velocity of the regulating wheel is controllable and is used to achieve any desired rotational speed of the workpiece. The angleαis usually taken from 1°to 5°and the larger the angle, the larger the longitudinal feed would be.
Whenαis taken as 0°, i.e., the two axes of the grinding and regulating wheels are parallel, there is no longitudinal feed of the workpiece.
5 Grinding Wheels
China precision machining Grinding wheels are composed of abrasive grains having similar size and a binder. The actual grinding process is performed by the abrasive grains. Pores between the grains within the binder enable the grains to act as separate single-point cutting tools.
These pores also provide space for the generated chips, thus preventing the wheel from clogging. In addition, pores assist the easy flow of coolants to enable efficient and prompt removal of the heat generated during the grinding process.
Grinding wheels are identified based on their shape and size, kind of abrasive, grain size, binder, grade (hardness), and structure.
Shape and size of grinding wheels. Grinding wheels differ in shape and size, depending upon the purpose for which they are to be used. include the following types:
1)Straight wheels used for surface, cylindrical, internal, and centerless grinding.
2)Bevelled-face or tapered wheels used for grinding threads, gear teeth, and the like.
3)Straight recessed wheels for cylindrical grinding and facing.
4)Abrasive disks for cutoff and slotting operations. (thickness 0.02 up to 0.2in. (0.5 to 5mm)).
5)Cylinders, straight cups, and flaring cups are used for surface grinding with the end face of the wheel.
The main dimensions of a grinding wheel are the outside diameter D, the bore diameter d, and the height H. These dimensions vary widely, depending upon the grinding process for which the wheel is to be used.
Kind of abrasive. Grinding wheels can be made of natural abrasives such as quartz, emery, and corundum or of industrially prepared chemical compounds such as aluminum oxide or silicon carbide (known as carborundum).
Generally, silicon carbide grinding wheels are used when grinding low-tensile-strength materials like cast iron, whereas aluminum oxide wheels are employed for grinding high-strength metals such as alloy steel, hardened steel, and the like.
Grain size of abrasive used. As you may expect, the grain size of the abrasive particles of the wheel plays a fundamental role in determining the quality of ground surface obtained.
The finer the grains, the smoother the ground surface is. Therefore, coarse-grained grinding wheels are used for roughing operations, whereas fine-grained wheels are employed in final finishing operations.
The grade of the bond. The grade of the bond is actually an indication of the resistance of the bond to pulling off the abrasive grains from the grinding wheel. Generally, wheels having hard grades are used for grinding soft materials and vice versa.
If a hard-grade wheel were to be used for grinding a hard material, the dull grains would not be pulled off from the bond quickly enough, thus impeding the self-dressing process of the surface of the wheel and finally resulting in clogging of the wheel and burns on the ground surface.
In fact, the cutting properties of all grinding wheels must be restored periodically by dressing with a cemented carbide roller or a diamond tool to give the wheel the exact desired shape and remove all worn abrasive grains.
Structure. Structure refers to the amount of void space between the abrasive grains. When grinding softer metals, larger void space are needed to facilitate the flow of the removed chips.
The binder. Abrasive particles are bonded together in many different ways. These include bond, silicate, rubber, resinoid, shellac, and oxychloride. Nevertheless, the bond is the most commonly used one.
China precision machining In fact, the standard marking system is employed for distinguishing grinding wheels, by providing all the preceding parameters in a specific sequence.